Test and measurement instruments, such a volt/ohmmeters, have been in common use for many years. Conventional volt/ohmmeters include a pair of test terminals, designated positive (“+”) and negative (“−”), that are connected to test points through respective wires or “leads.” Each lead generally has a respective plug at one end that plugs into one of the test terminals of the volt/ohmmeter, and a test probe, alligator clip, or other structure that is adapted to make electrical contact with a test point.
For AC or DC voltage measurements, the leads are connected to respective test points, and the voltage between the test points is measured by the instrument. For resistance measurements, the volt/ohmmeter passes a current between the two leads, and the voltage between the two leads is measured. The resistance in ohms is then determined as the ratio of voltage-to-current. In practice, the resistance is measured simply by measuring the voltage since the current is generally constant. The measured voltage, or, more correctly, the measured voltage calibrated in resistance, can then be read by a user on an analog pointer-type meter or on a digital display.
Conventional two-wire volt/ohmmeters are very satisfactory in most applications. However, under some circumstances, they do not provide sufficiently accurate resistance measurements. For example, when the resistance to be measured is very low, the resistance of the leads in relation to the resistance to be measured can be unacceptably large. Under these circumstances, a significant portion of the voltage between the leads as measured by the instrument can result from voltage drops in the leads rather than across the resistance to be measured. The instrument will then provide an erroneous resistance measurement. For example, assume that the resistance to be measured is actually 0.5 ohms, and each lead has a resistance of 0.2 ohms. If the volt/ohmmeter supplies a current through the leads of 100 mA (i.e., 0.1 amp), the voltage between the two leads as measured by the instrument would be 0.09 volts (i.e., 0.1*0.9). The instrument would then calculate the resistance to be measured at 0.9 ohms (i.e., 0.09/0.1) when, in fact the resistance to be measured is really 0.5 ohms.
To alleviate this problem, four-wire ohmmeters have been developed. In a four-wire ohmmeter, each of two test points is connected to the instrument through two separate leads. One of the leads in each pair carries the current to or from the resistance to be measured, and the other lead in the pair measures the voltage on one side of the resistance to be measured. Any voltage drop in the lead carrying the current is not reflected in the reading since the voltage-measuring lead measures the voltage at the resistance to be measured rather than at the junction between the instrument and the current-carrying lead. And since the input impedance of the resistance measuring circuit is generally very high, very little current is carried by the voltage-measuring lead, thus making any voltage drop across the lead nominal. An example of a four-wire ohmmeter is described in U.S. Pat. No. 5,508,621 to Wong, which is incorporated herein by reference.
Insofar as four-wire ohmmeters have four separate leads, the ohmmeter itself must have four test terminals. These test terminals are commonly known as the high (“HI”) terminal, the Sense HI terminal, which is connected to the same test point as the HI terminal, the low (“LO”) terminal, and the Sense LO terminal, which is connected to the LO terminal. The HI terminal is generally connected to a current source, and the LO terminal is generally connected to a corresponding current sink. The Sense HI terminal is connected to the positive input of a voltage measuring device, and the Sense LO terminal is connected to the negative input of the voltage measuring device. These test terminals have typically been in the form of cylindrical apertures or “jacks” that are adapted to receive respective “banana plug” connectors. Thus four banana plug jacks have typically been required in a four-wire ohmmeter.
One approach to reducing the number of jacks in a four-wire ohmmeter is described in the previously mentioned patent to Wong. The ohmmeter described in the Wong patent uses two banana plugs physically connected to each other through an insulator. Each of the banana plugs has two axially spaced contacts, which are connected to respective leads. The two-contact banana plugs are inserted into respective jacks in the ohmmeter, and each jack includes a pair of axially spaced contacts that mate with respective contacts of the banana plug.
The connector shown in the Wong patent has the advantage of reducing the number of jacks required by the ohmmeter, and it is also structured to be compatible with conventional two-wire leads, albeit without the benefit of a four-wire measurement. Although this connector represents a significant improvement in the art, it is nevertheless less than ideal in some situations. For example, insofar as the contacts of the banana plug are exposed, their contact surfaces can more easily become contaminated, which may undesirably increase their connection resistance. Also, it is possible for the connector shown in the Wong patent to make incorrect connections to the ohmmeter. For example, if the banana plugs are not inserted a sufficient distance into their respective jacks, the end contact of the banana plug may bridge the inner and outer contacts of the jack. In such case, the ohmmeter would provide a resistance measurement, but it would not be apparent that the measurement being made is essentially using only two rather than four wires.
There is therefore a need for an improved connector for use with a four-wire ohmmeter.